Process for scrubbing gases

ABSTRACT

Scrubbing process for separating vapors, liquid and solid particles from gas by a scrubbing liquid. A spout directs a full stream of the scrubbing liquid at a grid which comprises a multiplicity of spaced apart rods for forming films of the scrubbing liquid. The plane of the grid is transversely disposed with respect to the gas stream to be purified. The gas stream is passed through the films of scrubbing liquid for cleaning.

Unite States Patent 1 Stocchi et a1.

[ 51 May 8, 1973 [54] PROCESS FOR SCRUBBING GASES [75] Inventors:Virgilio Stocchi, Mestie; Antonio Cavraro, Porto Marghera; CarloMorslani, Saronno; Dario Vio, Porto Marghera, all of Italy [73]Assignee: Montecatini Edison S.p.A., Milan,

Italy [22] Filed: July 15, 1971 21 App1.No.: 163,032

Related [1.5. Application Data [62] Division of Ser. No. 873,822, Nov.4, 1969, Pat. No.

[30] Foreign Application Priority Data Nov. 7, 1968 Italy ....23393 A/68Sept. 29, 1969 Italy ..22639 A/69 [52] US. Cl ..55/64, 55/240 [51] Int.Cl. .Bllld 53/00 [58] Field of Search ..55/63, 84, 90, 238,

Primary Examiner-Reuben Friedman Assistant ExaminerR. W. BurksAttorneyI-Iubbell, Cohen & Stiefel [57] ABSTRACT Scrubbing process forseparating vapors, liquid and solid particles from gas by a scrubbingliquid. A spout directs a full stream of the scrubbing liquid at a gridwhich comprises a multiplicity of spaced apart rods for forming films ofthe scrubbing liquid. The plane of the grid is transversely disposedwith respect to the gas stream to be purified. The gas stream is passedthrough the films of scrubbing liquid for cleaning.

1 Claim, 7 Drawing Figures Patntd V May 8, 1973 I 1 V \\\M 4 V 5 llllnvug m V v 7 I 0 3 Ill. 7 n

'3 Sheets-Sheet l Patented May 8, 1 1973 3 Sheets-Sheet :FIG, 4;

FIG; '5;

Patented May ,1913 3,731,457

3 Sheets-Sheet 5 FIG. 6.

PROCESS FOR SCRUBBING GASES CROSS-REFERENCE TO RELATED APPLICATION Thisapplication is a division of co-pending application Ser. No. 873,822,filed Nov. 4, 1969 for Apparatus for Scrubbing Gases now U.S. Pat. No.3,01 1,592.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention pertains to the field of scrubbing apparatuses and methods forremoving vapors, liquid and solid particles from gases.

2. Description of the Prior Art The problem of removing fine powders,smokes, vapors mists and the like from the gases discharged byindustrial processes as well as the problem of purifying the gasesobtained or employed in the chemical processes, has been recognized fora long time in the prior art. Many methods and apparatuses have beenthus developed and improved, to the end of achieving better results asto the scrubbing efficiency and to the degree of removal of the fineparticles dispersed in such gases. The most suitable operationalconditions for the separation of a gas contained in a gaseous mixture byabsorption into a liquid, and so on.

However, all of the known scrubbing apparatuses possess variousdisadvantages, as for instance, the substantial pressure drop exhibitedby the gaseous stream while flowing through such apparatuses. So far,this drawback has been overcome by suplying the scrubbing apparatuseswith additional blowers. l lowever, the use of those blowers results inother drawbacks. For example, if the blowers are not carefullyregulated, which is costly, the pressure in the scrubber may risesharply, to the detriment of the acetylene making process. Otherwell-known scrubbing ap-- SUMMARY OF THE INVENTION We have now inventeda scrubbing process for separating vapors, liquid and solid particlesfrom gas by a scrubbing liquid, which comprises (1) a peripherallyextending wall which defines a longitudinally extending chamber; (2) agrid disposed along a plane transversely of the longitudinal extent ofthe peripheral wall and comprising a multiplicity of spaced apart rodsfor formproducing acetylene by hydrocarbon pyrolysis, including films ofthe scrubbing liquid in the spaces between adjacent spaced apart rods;and (3) a spout for directing a full stream of the scrubbing liquid atthe grid.

The gas to be treated is caused to flow in the same direction as oroppositely to the full stream of the scrubbing liquid. This liquidstrikes the grid along a direction substantially perpendicular to thegrid thus producing a liquid film substantially parallel to the grid andperpendicular to the flow direction of the gas. While passing throughthe liquid film, the liquid extracts the particles finely suspended ordispersed in the gas, as for example the carbon black particlescontained in the raw gas obtained by pyrolysis of hydrocarbons. In thiscase, the energy necessary for extracting the particles, instead ofbeing supplied by the gas to be treated, is provided by the scrubbingliquid. The stream ejected by the spout is full, i.e., is not atomizedor finely divided. In fact, we have found that the scrubbing process ofour invention is suitable for highly reducing, in comparison with theknown scrubbing processes the pressure drop exhibited by the gaseousflow while passing through the scrubber, avoiding the need for blowersheretofore necessary to supply the gas with the energy sufficient tobalancethe pressure drop and thus overcoming all the drawbacks relatedto the use of such blowers.

We have found that the process of our invention allows high degree ofscrubbing of the fine particles suspended or dispersed in the gas to betreated, as well as of vapors dissolved therein. Furthermore, thisscrubbing action occurs in our scrubber under the safest operationalconditions, without formation of any explosive mixture.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectionalview of a scrubbing apparatus in accordance with the present invention;

FIG. 2 is a sectional view taken along the lines 22 of FIG. 1;

FIG. 3 is a plan view of a spoke used to form the grid of an apparatusin accordance with the present invention;

FIG. 4 is a sectional view taken along the line 4-4 of FIG. 2;

FIG. 5 is a diagrammatic illustration of a plant for ing scrubbingapparatuses in accordance with the present invention;

FIG. 6 is a fragmentary view of the upper portion of cooling-scrubbingtower embodying the present invention;

FIG. 7 is an enlarged fragmentary view showing the central portion ofFIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsin detail and particularly to FIGS. 1, 2, 3, and 4, a scrubbingapparatus is shown, the apparatus including a vertically extending,elongated cylindrical side wall 1 provided with a grid 21, preferablymade up of a plurality ofspoke-like elements 3. As shown and preferredthe spoke-like elements (hereinbelow simply referred to as spokes)extend horizontally and radially inward from a cylindrical boss 4 onwall 1, thus defining a plane which is substantially perpendicular tothe vertical axis a--a of wall 1. The wall 1 defines a longitudinallyextending chamber 2. As stated, spokes 3 are fastened to the wall 1 bymeans of an annular boss 4. Boss 4 is preferably removably attached towall to facilitate the mounting and removal of said spokes 3. Saidchamber 2 is further provided with spout 5, said spout being disposedtherein and fastened thereto .by means of suitable supports 7. The axisof spout 5 is preferably disposed on the axis aa of the chamber 2. Spout5 directs a full stream of a suitable scrubbing liquid, for instance,water, against grid 21. This stream of scrubbing liquid leaves nozzle 5at a high velocity and strikes the center of the grid perpendicularly.Upon impinging on the grid center 6, the stream disperses or spreads outinto a plurality of liquid films or laminae, each being substantiallyperpendicular to axis a-1 of chamber 2 and each extending between orbridging the space between adjacent spokes.

The gas to be purified is caused to flow in an axial direction insidechamber 2, thus perpendicularly impinging on the liquid films and givingover to the films the fine particles suspended or dispersed in the gasor vapors dissolved therein. As indicated by the arrows of FIG. 1, thegas to be treated is caused to flow in the same direction as the fullstream of the scrubbing liquid ejected from spout 5.

The scrubbing liquid that leaves the apparatus (through a liquid outletthat is not shown in FIG. 1) therefore holds the material taken awayfrom the gaseous stream. As shown in FIGS. 3 and 4, each spoke 3preferably has a circular cross section. In addition, each spoke has atapered inner end 17.

The number of the spokes forming the grid can widely vary, depending onthe dimensions of the scrubbing apparatus. The spoke length and itscross-section diameter can also vary within wide ranges as well as thewidth of the taper angle a, which, however, is generally comprisedbetween 5 and 13. We have found that excellent results are obtained bymeans ofa grid having a ratio between the solid section i.e., .the areaactually covered by spokes 3, and the void section, i.e., the area ofthe entire cross-section 2-2, smaller than 70 percent and preferablyranging between and 70 percent. Grid 21 has a center 6 defined by themeeting point of the tapered ends 17 of the spokes 3 or it can bedefined by a small cylinder 18 to which the tapered ends 17 of spokes 3are fixed by suitable means. Although only one spout and only one gridare shown in the foregoing figures, it is however obvious that severalspouts and several grids can be employed for the same apparatus, withoutdeparting from the spirit of our invention.

We have further found that the best results, particularly with regard toreducing pressure drop and to the scrubbing efficiency are obtained whenthe apparatus has the hereinbelow defined covering of the spout on theimpact surface," smaller than 100 percent and preferably comprisedbetween percent and 85 percent.

By covering of the spout on the impact surface, we mean the percentageratio between the impact surface which is really impacted by the fullstream of liquid, s, and the projection of this stream on the plane ofsaid impact surface, S, which projection coincides with the crosssection surface of the full stream of liquid.

As shown in FIGS. 2 and 7, circle 19 defines the area of impact of thestream with the plane of grid 21. The portion of the plane of grid 21actually impacted by the full stream of liquid is the sum of all thetriangles 20 which are within said circle 19. The covering of the spouton the impact surface" above referred to is thus given by the expression(s/S) 100.

As previously stated, the gas to be purified is caused to flow either inthe same direction as or oppositely to the full stream of scrubbingliquid injected by the spout 5. The velocity of the gas, measured at thegrid, generally ranges between about 30 and 50 m/sec. According to thekind and the flow rate of the gas to be treated, the apparatus of thisinvention can be utilized alone or in connection with other alreadyknown scrubbing apparatuses, or several apparatuses of the presentinvention can be utilized at the same time by connecting them within thesame tower in parallel for increasing their flow rate, or in series forincreasing their efficiency, as will be more fully describedsubsequently.

Furthermore, we have experimentally found a relationship between thescrubbing efficiency achieved by the apparatus of our invention and thespecific consumption of the scrubbing liquid. The relationship can beexpressed as follows:

stages; Pi is the concentration (in mg/Nm) of the I material to bescrubbed off e.g. carbon particles); Pu is the concentration (in mg/Nm)of the particulate material stillremaining if the gas leaving theapparatus; m/G is the specific consumption (in KglNm of the scrubbingliquid per volume unit of the treated gas; H stands for the specificenergy of the full stream (in Kgm/kg.); ,8 is a constant depending onthe features of the spout; y is a constant depending on the kind ofmaterial to be scrubbed off from the gas (dimensions of the particles,their wettability, composition, etc.), (in' Nm /kgm); A is a constantdependingon the type of apparatus (number of spokes, shape of spokes andso on). The expression (1 m/G H '[3 2 '7) is to be calculated for eachstage. As used herein the term stage means the step in which the gas isscrubbed in one apparatus of the invention or in more than one apparatusof the invention but connected in parallel, as will be more fullyunderstood subsequently.

Referring now to FIG. 5, a plant for producing clean acetylene isdiagrammatically illustrated. The plantincludes a reactor 10 forproducing acetylene from hydrocarbons which reactor is well known in theprior art, a cooling-scrubbing tower 8 having a cooling section 50 and ascrubbing section60, the latter being stacked or superimposed on top ofthe former. In addition, the plant of FIG. 5 includes a separator 9 forremoving any entrained water particles from the acetylene gas beingtreated.

More specifically, and as shown in FIG. 5, the reactor 10 is connectedto the bottom of the tower 8 (especially to the bottom of the coolingsection 50 of said tower) by means of a duct or pipe 11 for introducingacetylene into the bottom of the tower). Neat the top of the coolingsection 50 of the tower 8 is a nozz le 52 for introducing in a downwarddirection a cooling liquid such as water supplied to the nozzle by awater pipe 12. Thus, as the acetylene moves upwardly through the coolingportion 50 of the tower 8 its temperature will be reduced and there willbe some incidental scrubbing.

As already noted, a scrubbing section 60 of tower 8 is stacked on top ofthe cooling section 50. As shown in FIG. 5, the scrubbing section 60 ismade of two scrubbing stages 62 and 64, each of them here shown asincluding one apparatus 82 and 91 in accordance with the presentinvention as shown in FIG. 1. The apparatuses are connected in series bymeans of baffles 66, 68, 69 and 70. As shown the apparatuses are alldisposed at an angle to the horizontal although other arrangements maybe employed. The grids 72 and 74 of the two series connected scrubbingapparatuses are both arranged perpendicular to the axis a-a of the apparatuses 82 and 91. Each of them is constructed substantially the sameas a grid 21 of FIG. 1. As shown, each of the apparatuses is providedwith a spout 14 which is adapted to direct a full stream of water attheir respective grids 72 and 74 and in a direction substantiallyperpendicular to the plane of the grids. As shown in FIG. 1 it will benoted from the serpentine arrowed line running through the drawing ofthe two scrubbing stages that the acetylene gas to be scrubbed impingesthe grids 72 and 74 in the same direction as the streams from the spouts14.

Each of the stages 62 and 64 operate substantially identically to thescrubber of FIG. 1. Thus an increased amount of particulate material andgas vapors can be removed from the acetylene through the seriesconnected two-stage arrangement of FIG. 5 as the relatively cleanacetylene leaving the lower stage 62 is scrubbed a second time in theupper stage 64 to thereby have thegas existing from the tower 8 throughthe duct 16 in a very clean or pure condition. Desirably the twicescrubbed gas leaves the tower .8 through a duct 16 and is then subjectedto the action of a suitable separator 9 which is a cyclone orcentrifugal separator,

grid was about percent. The covering of the spout on the impact surface"was about 65 percent.

A 250 mg/Nm content of carbon black was detected at the gas outlet fromthe first stage while a content of carbonblack of 65 mg/Nm was measuredat the gas outlet from the second stage. The total pressure dropsuffered by the gas through both stages was of mm of water.

In another test run of the apparatus of the same FIG. 5, 1,400 Nm /h ofraw acetylene obtained by pyrolysis of methane in reactor 10, and havingcarbon black content of 950 mg/Nm, were passed through column 8. In eachstage the pressure of the scrubbing water was 25 kg/cm its flow rate 4,5m/ h and the velocity of the full stream of water about m/sec; the voidsection of the grid was 300 cm the number of the grid spokes 24, eachspoke having a diameter of 14 mm. and taper angle a of 13; the ratiobetween the solid and the void section of the grid was about percent.The covering of the spout on the impact surface was about percent.

A 250 mg/Nm content of carbon black was detected at thegas outlet fromthe first stage while a content ofa carbon black of mg/Nm was measuredat the gas outlet from the second stage. The total pressure dropsuffered by the gas through both stages was less than 1,000 mm of water.

In a further test run of the apparatus of the same FIG. 5, 1,400 Nm lhof raw acetylene obtained by pyrolysis of methane in reactor 10, andhaving carbon black content of 950 mg/Nm", were passed through column 3.In each stage the pressure of the scrubbing water was 25 kg/cm its flowrate 4,5 m /h and the velocity of the full stream of water about 65m/sec; the: void section of the grid was 300 cm the number of the gridspokes 24, each spoke having a diameter of 4 mm. and taper angle a of 5;the ratio between the solid and the void section of the grid was about20 percent. The "covering of the spout on the impact surface" was about30 percent.

A 250 mg/Nm content of carbon black was detected at the-gas outlet fromthe first stage while a content of carbonblack of nig/Nm was measured atthe gas outlet'from the second stage. The total pressure drop.

suffered by the gas through both stages was less than 20 for removingany entrained water particles. Such water particles can be drained fromthe separator 9 through a drain 78. g

It will be obvious .from a perusal of FIG. 5 that the cooling water canbe removed from the cooling section 50 by a drain 76 at the bottom ofthe tower 8 and the scrubbing water introduced by the spouts 14 in boththe lower and upper scrubber sections can be removed by drains 15 nearthe bottom of the baffle plates 68 and 69.

In a test run of the apparatus of FIG. 5, 1,400 Nm /h of raw acetyleneobtained. by pyrolysis of methane in reactor 10, and having carbon blackcontent of 950 mg/Nm were passed through column 8. In each stage thepressure of the scrubbing water was 25 kg/cm its flow rate 4.5 m lh andthe velocity of the full stream of water about 65 m/sec; the voidsection of the grid was 300 cm*, the number of the grid spokes 24, eachspoke having a diameter of 8 mm. and taper angle a of' 10; the ratiobetween the solid and the void section of the mm of water.

As previously stated, to increase scrubbing efficiency, two or morescrubbing devices can be connected in series; to increase scrubbingcapacity two or. more scrubbing devices can be connected in parallel.Clearly, arrangements can be desirably made in which a plurality ofscrubbing devices are arranged in a plurality of series connectedstages, each stage comprising a plurality of parallel scrubbing devices.Such an arrangement, which displays both an increase in efficiency andin capacity, is shown in FIG. 6.

Referring now to FIG. 6, a fragmentary view of the upper portion of acooling-scrubbing tower 8 is shown. The scrubbing section 60 is made oftwo series connected scrubbing stages 62 and 64, each of which consistsof five parallel connected apparatuses of the invention whichapparatuses are designated82, 83, 84, 85 and 86; and 87, 88, 89, 90 and91, respectively. The two stages of parallel connected apparatuses areconnected in series by means of baffles 66, 68, 69 and 70 as in FIG. 5.The apparatuses are all disposed at an angle to the horizontal. Thegrids 72 and 74 are constructed the same as grid 21 of FIG. 1 and arearranged perpendicular to the axis a-a of apparatuses 82, 83, 84, 85,86, 87, 88, 89, 90 and 91. As shown, each of the ten apparatus isprovided with a spout 14, which directs full stream of water at theirrespective grids, in a direction substantially perpendicular to theplane of the grids.

Similar to the construction of FIG. 5, the acetylene gas to be purifiedimpinges against the grids in the same direction as the stream from thespouts 14. In one experimental run of the apparatus of FIG. 6, 7,500 Nm/h of raw acetylene obtained by pyrolysis of methane and having carbonblack content of 1,000 mg/Nm, were passed through column 8. In eachstage the pressure and the flow rate of the scrubbing water' were 25kg/cm 24 m /h respectively (in all for the five apparatuses) and thevelocity of the full stream of water about 65 m/sec; the total waterflow rate was 48 m /h for both stages.

In each apparatus the void section of the grid was 300 cm, the number ofspokes 24, each spoke having a diameter of 6 mm and taper angle a of 8;the ratio between the solid and the void section of the grid was 35percent. The covering of the spout on the impact surface is about 55percent.

A carbon black content of about 200 mg/Nm was detected at the gas outletof the first stage while a content of about 50 mg/Nm was measured at theoutlet of the second stage.

The total pressure drop suffered by the gas through both stages was lessthan 50 mm of water.

In another experimental run of the apparatus of FIG. 6, 12,500 Nm lh ofraw acetylene obtained by pyrolysis of petrol and having carbon blackcontent of 3,500 mg/Nm were passed through column 8. In each stage thepressure and the flow rate ofthe scrubbing water were 25 kg/cm 40 m /hrespectively (in all for the five apparatuses) and the velocity of thefull stream of water about 65 m/sec; the total flow rate was m /h forboth stages.

In each apparatus the void section of the grid was 300 cm the number ofspokes 24, each spoke having a diameter of 8 mm and taper angle a of 10;the ratio between the solid and the void section of the grid was 50percent. The covering of the spout on the impact surface" is about 65percent.

A carbon black content of about 10 mg/Nm was measured at the gas outletof the second stage.

The total pressure drop suffered by the gas through both stages wasabout 360 mm of water.

The advantages of the present invention, clearly evidenced by the abovedescription and examples, can be summarized as follows: low pressuredrop and high scrubbing efficiency, as have been discussedjconstructionsimplicity due to the reduction in required pumping or blowerapparatuses and controls therefor; and low services cost which isexpected from the reduction in blower apparatuses and related controls.

Modifications and variations can, of course, be made without departingfrom the spirit and scope of this invention.

Having thus described our invention, what we desire to secure by LettersPatent and hereby claim is:

l. A process for purifying raw acetylene produced by pyrolysis ofhydrocarbons, comprising the steps of forming in spaces between spacedapart radially extending rods of a grid-like structure films ofscrubbing liquid; and passing said raw acetylene through said films.

